I can honestly tell you that a 12V 30A power supply handles the current requirements of the 12" just fine and heats up without issue. I hit 120 degrees on the bed and 250 on the hot end in around 5-6 min.

It may push it near it's limit but it handles it just fine :-)

and to add to what printbus said, The 12" printbed comes stock 12 12AWG wire, and i used 12AWG from the power supply to the relay